The p53 protein has been described as the central node of a regulatory circuit that monitors signaling pathways from diverse sources, including DNA damage responses, abnormal oncogenic events and everyday normal cellular processes. Inactivation of the p53 pathway is a common, if not universal, feature of human cancer. Activation of p53 function is crucial for suppressing tumor growth in many tumor types. It is well accepted that Mdm2 and its related protein Mdmx (also called Mdm4) play a major part in the scope of negatively regulating the tumor suppressor p53 activities in vivo; however, it is becoming more apparent that the roles of Mdm2 and Mdmx in regulating p53 is not as simple as once thought. For the past several years, our lab has made significant contributions to understand this dynamic p53/Mdm2 pathway. For example, we discovered the molecular basis for the differential effects of Mdm2-dependent ubiquitination on p53 (Li et al., Science, 2003); we identified the deubiquitinase HAUSP in controlling Mdm2 self-ubiquitination and p53 stabilization (Li et al., Mol. Cell, 2004; Li et al., Nature, 2002); our recent studies demonstrated Tip60 as a crucial cofactor of p53 in differentially regulating cell cycle arrest, apoptosis and the p53-Mdm2 feedback loop (Tang et al., Mol. Cell, 2006). Notably, these original observations from our laboratory have been supported or verified in the studies by others in different systems (review in Brooks and Gu, Mol. Cell, 2006; Tyteca et al., Mol. Cell, 2006). Further mechanistic analysis of these dynamic interplays should elucidate the precise mechanisms for both Mdm2 and Mdmx in tumorigenesis. This study includes the following four aims and represents the major research interest in our laboratory. In Aim 1, we will investigate the mechanisms by which Mdm2 and Mdmx repress p53-mediated transcriptional activation. In Aim 2, we will examine the regulatory role of p150, a newly identified Mdm2-associated protein, in controlling Mdm2 activities. In Aim3, we will elucidate the precise role of Mdm2-mediated modification in modulating p53 function. In Aim 4, we will define the precise role of Mdmx-mediated effects on p53 by elucidating the components of Mdmx associated nuclear protein complexes. We have also obtained significant preliminary results to support the feasibility of each aim. These studies will yield crucial new insights regarding how to target these two validated oncoproteins in cancer therapy.